[concurrency-interest] Strange behaviour when sorting an uniform array

Lukas Krecan lukas.krecan at gmail.com
Tue May 25 04:15:25 EDT 2010


Hi,
   I am quite sure that it has nothing to do the synchronization. The
problem remains even if synchronized keyword is removed. Besides, the method
was used only for generating values, which works well. Problem is with
sorting. It works well on random values, but runs for ages on uniform array.


On Tue, May 25, 2010 at 10:00 AM, Kasper Nielsen <kasper at kav.dk> wrote:

> Hi Lukas,
>
> try removing the synchronized keyword in your op.
>
> Taken from the extra166y.Ops javadoc:
>
> In addition to stated signatures, implementations of these interfaces must
> work safely in parallel. In general, this means methods should operate only
> on their arguments, and should not rely on ThreadLocals, unsafely published
> globals, or other unsafe constructions. Additionally, they should not block
> waiting for synchronization.
>
> If you want to return random numbers you should use the ThreadLocalRandom.
>
>
> - Kasper
>
>
> On 25/5/2010 07:9, Lukas Krecan wrote:
>
>> Hi,
>>    I have been playing with extra166y library and found that when I try
>> to sort an uniform array (full of ones) it takes ages. With older
>> version of the library it even threw StackOverflowError. After update it
>> runs for minutes (I always kill the process after while so I do not know
>> if it ends successfully).
>>
>> Probably it's nothing new for you, maybe it's expected behaviour. But
>> for me it's confusing and I just wanted to let you know about the issue
>> in case you were not aware of it.
>>       Best regards
>>         Lukas Krecan
>>
>> -------------------------------------
>> package net.krecan.forkjoin;
>>
>> import java.util.Random;
>>
>> import jsr166y.ForkJoinPool;
>> import extra166y.Ops;
>> import extra166y.ParallelLongArray;
>>
>>
>> public class SortTest {
>>     private static final int THREADS = 2;
>>     private static final int SIZE = 40000000;
>>
>>     public static void testSort()
>>     {
>>         ForkJoinPool fjPool = new ForkJoinPool(THREADS);
>>         ParallelLongArray pa = ParallelLongArray.createEmpty(SIZE,
>> fjPool);
>>         createData(pa);
>>         System.out.println("Sorting "+pa.summary());
>>         long start = System.currentTimeMillis();
>>         pa.sort();
>>         System.out.println("Done in
>> "+(System.currentTimeMillis()-start)+"ms.");
>>         System.out.println(pa.summary());
>>
>>
>>     }
>>     private static void createData(ParallelLongArray pa) {
>>         long start = System.currentTimeMillis();
>>         System.out.println("Creating data using "+THREADS+" threads.");
>>         pa.setLimit(SIZE);
>>         final Random rand = new Random();
>>         pa.replaceWithGeneratedValue(new Ops.LongGenerator(){
>>             public synchronized long op() {
>>                 return 1L;//rand.nextLong();
>>             }
>>         });
>>         System.out.println("Done in
>> "+(System.currentTimeMillis()-start)+"ms.");
>>     }
>>
>>     public static void main(String[] args) {
>>         testSort();
>>     }
>>
>> }
>>
>>
>>
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>
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